DENIERS SAY:

Water vapor is a greenhouse gas, and there’s a lot more of it in the atmosphere. So why all the focus on CO2?

SCIENCE SAYS:

Carbon dioxide is like a control knob on our planet's thermostat. More carbon pollution = more global warming.

Here's what's most important: If you increase the amount of carbon pollution, the planet warms. And humans have increased the amount of carbon dioxide by about 40% by burning dirty energy. Water vapor is important, too, but the details are a little technical. As carbon dioxide traps heat and the planet warms, two important things happen: 1) More water evaporates from the oceans and land, and 2) the atmosphere retains more water (because warm air can hold more moisture than cold air). Then, since water vapor is a greenhouse gas, it causes the temperature to go up even further. Studies show that the water vapor "feedback loop" roughly doubles the warming caused by carbon dioxide.

Additional info from the Center for Climate and Energy Solutions

About 150 years ago, John Tyndall deduced that water vapor absorbs more heat in the atmosphere than any other gas (PDF). He was correct. More than a century ago, Svante Arrhenius postulated that the amount of water vapor in the atmosphere changes in response to the amount of CO2 and amplifies the temperature effect of the CO2 (PDF). He, too, was correct.

Here’s how it works: As the amount of CO2 in the atmosphere rises, it traps more heat, causing the surface air temperature to rise a bit. The higher temperature evaporates more water from the surface of the ocean and land. Since warmer air can hold more water vapor, the amount of water vapor in the atmosphere continues to increase as long as the concentration of CO2 continues to rise. The extra water vapor traps heat in addition to that trapped by the rising CO2, amplifying the CO2 effect by about twofold. Scientists call this amplification a “positive feedback,” and it works in both directions: If atmospheric CO2 were to decrease, some water vapor would rain out of the atmosphere, creating a positive feedback that would amplify the cooling effect of CO2 removal.

The short “residence time” of water vapor in the atmosphere means that it cannot be said to drive global warming. Even if something spewed massive quantities of water vapor into the atmosphere, it would simply rain out within days, long before it had time to elevate the global temperature (it takes decades for heat to build up in the climate system). Therefore, only a driver that continues long term can cause climate change. The persistent heat-trapping activity of long-lived gases like CO2, methane, and nitrous oxide keeps extra water vapor aloft so that it can amplify warming. These gases stay in the atmosphere for a decade to several centuries before they are removed by natural processes.

In short, long-lived heat-trapping gases released by human activities — mainly CO2 — are driving global warming, and water vapor is responding and amplifying the initial warming by about twofold. This understanding dates back more than a century and has been confirmed through many theoretical advancements and modern atmospheric observations.